Improving both dewatering and paper sheet properties exiting the press section are two issues addressed in papermaking. The challenge with these two issues is that an improvement in dewatering at the press section, leading to an increase in the solids content exiting the press section, comes at the expense of sheet properties and the inverse is true as well. Various methods have been employed to address these issues.
A primary driver for dewatering a paper sheet is the application of mechanical pressure to the paper sheet at the press section, particularly at the press nip. More specifically, a paper sheet, which is supported in a press nip by one or more porous media structures, such as press fabrics, is subjected to mechanical pressure at the press nip(s) in the press section.
In the 1970's the relationship between applied pressure and nip residence time was expressed by Beck of Appleton Mills and Busker of Beloit as impulse, which was the product of the two components P (pressure)×t (time). Increasing the impulse typically improves dewatering during pressing and can be achieved by increasing the length of the press nip.
This understanding to extend the time under which pressure is exerted upon the paper sheet was applied first for paper grades that are considered to be flow controlled. The first presses with press nips of extended lengths were large diameter rolls (LDR), followed in 1981 by the first shoe press. Both the LDR and shoe press allowed for significant increases in nip residence time over which the applied pressure could act to dewater the paper sheet. Not only was crushing avoided, but sheet solids were increased compared to the best standard roll presses available.
There are, however, practical limitations to the rate of pressure development applied at the press nip(s), because too high a rate of pressure development will lead to sheet breakage, sheet disruption (crushing), or sheet marking.
Other technologies to enhance water removal were explored. The application of heat to the press section, for example, via steam showers, has improved mechanical removal of water from the press section as well. The application of heat raises water temperature and lowers its viscosity, thus making it easier to mechanically remove water from the sheet. Specifically, a further development not commercialized involves the application of heat directly in the press nip to create a displacement steam front which would not only reduce the viscosity of water, but the steam front as it passes through the sheet would physically displace additional sheet water. Improvements in dryness of up to 10 percentage points were seen with additional improvements in sheet properties. Practical considerations have kept such a process from commercialization.
Other means for fluid displacement have also been taught in the prior art. Air presses have been utilized to force air through the sheet to displace “free water” from the paper sheet. The same was true with other fluids such as foam.
A chemical approach to dewatering a paper sheet in a press section has not been so successful. For example, most chemical drainage aids used in the forming section have not been shown to work in the press section.
In addition, attempts to use soaps or compounds with quaternary amine compounds in pilot trials have resulted in limited success in increasing sheet dewatering during pressing and decreased sheet strength properties due to interference with hydrogen bonding of the cellulose fibers.
Moreover, water insoluble solvents have been introduced into the press nip to replace sheet water. These solvents increase sheet solids exiting the press nips because they displace free water in the paper sheet. Drying rates in the drying section are increased because the solvents are more easily evaporated in the dryer section. This technique is discussed in U.S. Pat. No. 4,684,440 issued to Penniman et al., which is herein incorporated by reference. However, while the mechanism appeared to work for certain light weight paper grades (50 gsm or less), environmental and safety considerations have prevented implementation of this technique.
Both sheet properties and sheet dewatering are affected by the press media structure. More specifically, the press media's Mean Flow Pore (MFP) size influences paper sheet properties. In particular, smaller pore size (denoting a “finer” structure) imparts greater sheet smoothness to the paper sheet in the press nip, a desired outcome. There are practical limitations to press fabric MFP size. Too small a MFP size can have an adverse affect on sheet dewatering, especially of heavier basis weight sheets that are considered to be flow controlled, specifically an increase in fabric flow resistance and an increase in hydraulic back pressure in the sheet at the press nip. In addition, too small of a pore size creates a potential for sheet disruption, sheet breakage, and sheet marking due to an increase in hydraulic pressure